Well, in some fanfictions, the Creepypasta elements might seep into the Miraculous Ladybug world by infecting the Kwamis. Imagine if Nooroo, the Kwami of Transmission, got influenced by a Creepypasta entity. This could lead to the Miraculous being used in really dark and unexpected ways, changing the entire nature of how the powers are supposed to be used in the original Miraculous Ladybug series.
A possible crossover could involve the Miraculous Ladybug characters getting trapped in a Creepypasta - like dimension. Marinette and Adrien would have to use their wits and powers not only to survive but also to find a way back home. The spooky and often dangerous nature of the Creepypasta world would put their relationship and their heroism to the test in ways that are very different from their normal battles against Hawk Moth.
One interesting crossover could be a Creepypasta villain somehow getting their hands on a Miraculous. For example, a Creepypasta character like Jeff the Killer might try to use the Cat Miraculous' power of destruction in a really chaotic way. It would create a whole new dynamic as the Miraculous Ladybug heroes would have to face a much more sinister and unpredictable enemy.
A crossover with the world of Sailor Moon would also be cool. The Miraculous holders could meet the Sailor Scouts. They could exchange their powers and fighting techniques. For example, Ladybug could show Sailor Moon how to use her yoyo in a more strategic way, and Sailor Moon could teach Ladybug about the power of love and friendship in her battles.
One interesting crossover could be the Ladybug and Cat Noir characters getting Autobot or Decepticon powers. Maybe Ladybug could use her Miraculous powers in combination with a Transformer's ability to change forms for some super - cool battles.
One interesting crossover could be that the villains from both universes team up. For example, Green Goblin from Spiderman 3 and Hawk Moth from Miraculous Ladybug might join forces. They could create chaos in both New York (from Spiderman) and Paris (from Miraculous Ladybug). This would force Spiderman and Ladybug to work together, which would be a really cool dynamic as they have different powers and fighting styles.
A cool crossover in the fanfiction could be a situation where the two universes are in danger simultaneously. Iron Man and Ladybug have to team up to fight villains from both their worlds. Ladybug's Lucky Charm ability could be really useful in unexpected ways when combined with Iron Man's strategic thinking. They might also exchange their knowledge about their respective technologies and magic. Iron Man could be fascinated by the Miraculous and try to understand its science, while Ladybug could learn some new ways of using her powers from Iron Man's engineering skills.
One interesting crossover could be that Ladybug and Spiderman team up to fight a common villain. Maybe the villain has the ability to create chaos in both Paris (where Miraculous Ladybug is set) and New York (Spiderman's territory). They could combine their unique powers. Ladybug's Lucky Charm and Spiderman's webs could be used in creative ways to outwit the enemy.
One interesting crossover could be the characters from both universes teaming up to fight a common enemy. For example, Spider - Man's web - slinging skills combined with Ladybug's Lucky Charm power could make for some really cool fight scenes. They could be facing a villain who has the ability to create chaos in both New York (Spider - Man's city) and Paris (where Miraculous Ladybug is set).
Creepypasta can add a sense of horror and the macabre to Miraculous Ladybug fanfiction. It might change the tone from the light - hearted and magical feel of the original show to something much darker. For instance, the characters could face more terrifying threats than just Hawk Moth.
Definitely Superman if it's a DC crossover. His super strength and ability to fly could add a whole new dynamic to the battles against Hawk Moth. He could also be a mentor figure to Ladybug and Chat Noir, teaching them about using their powers more responsibly on a global scale.
One interesting crossover could be the Avengers teaming up with the Miraculous heroes to fight a common super - powerful villain. For example, Thanos could be a threat to both the Marvel Universe and the world of Miraculous. The Avengers' brute strength combined with the Miraculous holders' unique powers would make for an epic battle.
Another theme is the power dynamics within the superhero world. How Ladybug's leadership as the main superhero affects the others, and how Chat Noir supports her. Fanfictions can explore different scenarios where they have to make difficult decisions regarding their powers and responsibilities, such as when they face a villain who is stronger than they expected and they have to figure out new strategies using their powers.